Smoke Domain fields are not editable
The smoke domain settings become uneditable if smoke simulation was baked.
Note that the file must be saved in order the baking to work:


  Baking can only been done once your .blend is saved. If your .blend has not been saved, the Smoke Cache panel will be disabled.


manual page

Press Free Bake button in the Cache rollout in domain settings (it's called like that only if simulation is baked). This will remove the calculated data with the next modification or frame change and it should be baked once more. However it'll be possible to continue editing any settings of smoke sim.